Fibrous webs, particularly low basis weight webs between 8 and 60 lbs. per ream (3,000 sq. ft.), for use ultimately as tissue, towel and napkin products, are fabricated conventionally by two alternate processes. The older wet laid process dispenses an aqueous slurry of pulped paper-making fibers, generally natural cellulose fibers, onto a moving foraminous support means, e.g., a fourdrinier wire, the aqueous medium being removed through the support means by vacuum means. The wet laid web is thermally dried and taken up on a parent roll. Because of the presence of water, the wet laid web fibers bond naturally to one another by means of hydrogen bonding. Such conventionally prepared webs are sometimes creped to improve feel and enhance absorbency. Similarly, the webs may be embossed to enhance softness and to provide a more aesthetic appearance.
The second, now conventional, yet relatively recent, process defiberizes cellulose pulp, the dry individual fibers being pneumatically transported to the dispensing means, and then dry laid (or air laid) onto the moving foraminous support means. Vacuum means below the support means is employed to ensure that the dry fibers remain on the web, which web has little inherent strength inasmuch as hydrogen bonds are not formed substantially in the absence of an aqueous medium. The dry, initially laid web is then sprayed with a synthetic bonding agent, such as a latex emulsion, preferably on both surfaces of the web. The bonding agent is cured by passing the thus treated web though a dryer, e.g., a through air dryer, before being taken up on a parent roll. These webs may also be creped and embossed.
Generally speaking, wet laid webs provide better wiping absorbency than dry laid webs. That is, under dynamic conditions of use, wet laid webs absorb liquid at a faster rate and retain the liquid thus absorbed better than their dry laid counterpart. The slower rate of wiping absorbency associated with dry laid webs is primarily due to the greater volume of interstitial voids existing within the reticulated structure of these webs. The greater void volume is occasioned by the larger size, on average, of individual pores, as well as by the greater degree of reticulation extant in the overall web structure. These interstitial voids provide a greater interstitial liquid holding capacity under static conditions, but permit absorbed liquid to be squeezed back out more easily under dynamic wiping conditions. The net result is an appeared slower rate of wiping absorbency for dry laid webs. The advantages of greater void volume or bulk present in dry laid webs are several. Less fiber per ream is required to fabricate the web. The greater void size provides greater static liquid holding capacity, and typically contributes to a softer feeling web. Finally, dry laid webs have greater wet strength than conventional wet laid webs, and, hence, do not break apart as readily during use. Thus, substantial improvement in wiping or dynamic absorbency characteristics would greatly improve product acceptance and usefulness.
Applicants have found that a pattern imprinted on at least one surface of a dry laid web wherein the compressed areas comprise at least 40% of the total imprinted area improves web absorption rate and enhances water retention during use. Heretofore, patterned webs, whether by imprinting or by embossing, have been produced to enhance the softness, improve the bulk, or to alter the strength and stretch characteristics of the web, as well as to impart an aesthetic design to the surface.
Embossing and imprinting as used in this application are distinguished below, although terminology in the art is not uniform and does overlap.
In conventional embossing, a raised pattern is formed on a portion of a first web surface (the raised portion), with corresponding depressions in the remaining portion of said web surface (the depressed portion). These discontinuities in the first surface of the web are occasioned by passing the web between two cylindrical rolls, one of which is resilient, the other being inextensible and having a plurality of bosses thereon. The bosses contact the second surface of the web forming the raised areas (and depressions) on the first surface in contact with the resilient roll. Typically, the raises comprise about 20 to 30% of the total first surface area, the depressions accounting for the remaining area. Surface depressions are also formed on the second surface where the bosses come into contact with the web, these depressions being in alignment with the raises on the first surface. Embossing of this nature may result in products having a raised area approaching 60%, as in U.S. Pat. No. 3,337,388 to Wosaba, which embossing improves softness, bulkiness and sponginess.
Imprinting, on the other hand, as the term is used herein, compresses certain portions of a web surface in intaglio, the other surface not being raised thereby. Necessarily, imprinting densifies the compressed surface portions substantially more than embossing.
Imprinting, as opposed to embossing, has heretofore been used in wet laid processing in conjunction with subsequent creping as disclosed in, for example, U.S. Pat. No. 4,191,609 to Trokhan and U.S. Pat. No. 4,125,659 to Klowak et al. In so doing the finally creped product exhibits greater softness, improved stretch and tensile strength, and enhanced bulk.
In dry formed webs surface imprinting, as hereinbefore defined, has been performed to improve the application of bonding agent to the web. In U.S. Pat. No. 4,127,637 to Pietreniak et al between about 15 to 40% of the web surface is compressed by cylindrical rolls prior to bonding, the pattern being then stabilized with binder to retain the differential density. The web is then creped to improve softness and bulk. Similarly, U.S. Pat. No. 4,135,024 to Callahan et al imprints one web surface, simultaneously applying binder to the other web surface, thereby enhancing permeation of the binder into said web. U.S. Pat. Nos. 3,692,622 and 3,776,807 to Dunning and Dunning et al respectively disclose spot bonding of dry laid webs wherein 5 to 40% of the web surface is imprinted. In Baker, Jr., U.S. Pat. Nos. 4,207,367 and 4,138,848, a plurality of highly compressed, narrow regions, preferably about 30% of the surface, separate the high loft regions, the high loft regions being bonded only partially.
Finally, embossing and imprinting are employed to laminate two webs together as disclosed in U.S. Pat. No. 3,867,225 to Nystrand.